1. Field of Invention
This invention relates to a scheme designed for counteracting subsynchronous resonance (SSR) phenomena in an AC power system comprising neutralizing the undesired SSR currents by means of a passive shunt device.
2. Description of Prior Art
In its most common form, SSR is a critical condition that can be reached by a subsynchronous frequency oscillation (SSO), this is a phenomenon in which the torsional system of a power plant prime-mover shaft interchanges energy with a series-compensated transmission network via the generator air gap, at one or more natural frequencies of the combined system.
Various SSR Countermeasures are known to the industry, the IEEE Power Engineering Society's SSR Working Group in its Transaction paper PAS-99 of Sep/Oct 80 entitled `Countermeasures to Subsynchronous Resonance Problems`, and later in its recent paper 91 SM350-9 PWRS `Readers's Guide to Subsynchronous Resonance` presents an updated list of countermeasure concepts proposed or applied.
In practice, a leading option has been found on `dynamic` or `active-type` means i.e. RL devices switched with back-to-back thyristor firing control. The `Static Var Generator` (also known as `Dynamic Stabilizer`), U.S. Pat. No. 4,438,386 to Gyugyi is an example of this approach.
A number of disadvantages of this apparatus can be listed as follows:
a) cost;
b) involves many components;
c) utilizes sophisticated detection and control means;
d) requires a non-standard transformer fabrication (U.S. Pat. No. 4,513,243 to Novak et al.);
e) thyristor switching produces undesired harmonics that lower the quality of the power supply;
f) its location at generator or step-up transformer terminals where short-circuit power duty is near maximum, minding b) it becomes clear the overall system reliability deterioration;
g) it can control a single SSR frequency at a time.
Another major active-type countermeasure is the so called `NGH Scheme` or thyristor switched resistor which has many of the forementioned disadvantages such as: a), b), c) (here a U.S. Pat. No. 4,607,217 to Bhargava indicates the detection means used in this scheme), e),and g) plus the fact that all this bulky equipment must be placed onto the Extra-High-Voltage (EHV)-insulated platform together with the series capacitor installation making the substation layout and wiring congested.
Insofar as passive filters are concerned, the cited IEEE references discuss or mention three types:
a) Static Blocking Filter (Navajo Filter);
b) Line Filter;
c) Parallel Filter;
b) and c) are only theoretical and consist of RLC elements connected in parallel with the capacitor bank either to block the SSR current as in b) or to by-pass the capacitor for a SSR frequency as in c). Neither one has ever been applied basically due to their substantial potential cost since they must be EHV-insulated apparatus of very low losses and one is required per SSR frequency. Conversely a) was applied at the Navajo Plant in northern Arizona; this is a series filter connected from the neutral end of the step-up transformer high side to save in insulation, yet requiring a non-standard design of such transformer, the filter is expensive since it must withstand both a severe short-circuit duty and full load, occupying besides considerable substation space due to its fairly large number of components.
It can be observed that no prior-art concept uses the principle of neutralizing the harmful subsynchronous currents by means of a shunt passive filter located in the EHV network.
A shunt filter element placed at an intermediate point 0 between the interacting generator and series capacitor (FIG. 1) has inherent potential properties to cope with SSR, some of which are listed below. Depending on system topology and number of frequencies in question, their effectiveness can be shown in a number of not mutually exclusive contexts such as:
a) the filter is a short circuit to ground at a given SSR frequency so as to completely decouple electrically the generator from the capacitor, breaking thus any possible energy interchange between them at that frequency.
b) the filter has, at a given SSR frequency, an admittance YOP whose magnitude is the conjugate of the capacitor-branch apparent one YOU as seen from the intermediate point 0, so that their parallel combination (YOP+YOQ) is nil or very small, forcing the subsynchronous current fed from the generator to be negligible;
c) still the filter has an admittance to ground at the concurrent supersynchronous frequency component so as to boost its always positive damping contribution at the common natural frequency of the generator's torsional oscillation.
One reason why this resourceful approach has never been formally attempted can be offered basically as follows: a self-standing shunt element in the system must present ideally zero admittance at synchronous frequency in order not to alter the AC supply. In a passive filter case this calls for an EHV-insulated parallel LC tank tuned at 60 Hz to block this frequency plus a `bleeding` stage comprising additional LC components connected in series with that blocking unit to render the required overall admittance at the SSR frequency.
A major drawback of this scheme lays in the fact that an EHV shunt capacitor is not a standard apparatus and moreover the filter-assembly MVA rating, plus its stringent design for low steady-state losses and attendant protective switchgear, turn it absolutely unfeasible from a cost standpoint.